Review of the early Mammal Brasilichnium and Brasilichnium-like tracks from the Lower Cretaceous of South America

摘要

The ichnogenus Brasilichnium Leonardi, 1981 is an early mammal track, presently recognized in several continents, but originally found in the aeolian deposits of Botucatu Formation in the Parana Basin, Brazil. The sandstones of the Botucatu Formation are generally fine-grained and well-sorted, containing no pebbles; its color can be white, yellowish or reddish, but more commonly it is pinkish. Nearly always, it is silicified and therefore compact and hard, making it a very suitable building material. These sedimentary rocks are interpreted as a Lower Cretaceous desert regarded as the largest paleodesert in the Earth's history. Although the aeolian environment is generally considered inadequate for track preservation, taphonomic events linked to moist dunes and microbial mats foster a good geological context for an early diagenization of the sediments. Brasilichnium is associated with a large ichnofauna of other mammal, theropod and ornithopod tracks, in addition to invertebrate traces. It is today a worldwide ichnogenus that allows an overview of the arid terrestrial ecosystems during Mesozoic. The presence of tritylodontids in the world after the end of the Triassic and up to the Late Cretaceous, and the possibility of attributing Brasilichnium and related ichnogenera to them, has been discussed at great length. It seems preferable, however, to suggest that these tracks can only belong to the Mammalia. Thus, these track-bearing deposits bear witness to small- and medium-size mammaliaform trackmakers; some of them would be remarkably larger than the coeval mammals represented by skeletal remains. This chronological discrepancy between ichnological and bone findings is not a surprising phenomenon: it is also known in other clades. A complete check of these tracks will have to be carried out. It would also be advisable to perform a review of all the tracks recently attributed to Brasilichnium world-wide. The Botucatu and Caiua ichnofaunas and, globally, the ichnogenus Brasilichnium, should be stressed as an important window into the arid terrestrial ecosystems during the Gondwanic Cretaceous. This study is the result of forty-four years of research in the Botucatu Formation by the first author.